| As lithium transiton metal phosphates cathode materials for lithium ion batteries,olivine-type lithium iron phosphate LiFePO4and monoclinic lithium vanadium phosphateLi3V2（PO4）3are attractive for stable structure, excellent cycle performance and high safety.However, their low electronic conductivity impedes their used as electrode materials. In thispaper, nano-sized LiFePO4/C composite was prepared by a two-step method, porousLi3V2（PO4）3/C composite was prepared though sol-gel-combustion method,Li3V2（PO4）3@C/Graphene composite was prepared by a modified Pechini method. Themicrostructure and morphology of the materials were investigated by XRD, SEM and TEM.The electrochemical performance and kinetics of the materials were investigated bygalvanostatic current charge-discharge, cyclic voltammetry and electrochemical impedancespectroscopy. This paper focuses on the effects of synthesis method on the structure,morphology and electrochemical performance of the materials.In Chapter1, a general introduction is given on following aspects: the history anddevelopment of lithium ion batteries, the structure features and working principle of lithiumion batteries, the recent development of usual cathodes for lithium ion batteries and somecommon methods for improving the performance of electrode materials.In Chapter2, the author mainly introduces the experimental raw materials, equipmentand methods used in the project of this thesis. A detailed description on the process of makinga coin cell is presented. The structural and electrochemical analyses methods are alsosummarized.In Chapter3, the olivine LiFePO4/C composite is successfully synthesized by a simpletwo-step method. In previous studies, sol-gel method has commonly been employed forpreparing nano-sized LiFePO4. In the sol-gel method, the precursors can be mixed at amolecular level, but it is difficult to control the carbon content for the coating. In a typicalsolid-state reaction, it is convenient to control the carbon content, but the size of the product isdetermined by the raw materials, which usually is large. To prepare nano-sized LiFePO4/Ccomposite with a controllable carbon content, the combination of the sol-gel method and thesolid-state method is proposed. The as-prepared LiFePO4/C exhibits a good electrochemicalperformance. At the rates of0.1,0.2,1,2, and5C, the as-prepared LiFePO4/C has specificcapacities of162,154,145,124, and107mAh g-1, respectively. It also shows an excellentcapacity retention, and no capacity loss is observed during50cycles. In Chapter4, porous Li3V2（PO4）3/C cathode materials are synthesized via a sol–gelcombustion method and present extremely excellent high-rate performance. Theas-prepared porous Li3V2（PO4）3/C exhibits an excellent specific capacity of105mAh g-1at100C rate between3.0and4.8V. This performance enhancement is attributed to the porousstructure with a thin carbon layer covering all the surfaces and the nano-sized particle, whichcould improve the ion diffusion and electron conductivity. The materials synthesized in thispaper are promising to be used for fast charging and discharging and the sol–gel-combustionmethod is a simple and universal way to prepare the high performance electrode materials.In Chapter5, a novel structured cathode material Li3V2（PO4）3@C/Graphene has beenprepared and a new reaction pathway is presented. It is first proposed that the graphene oxidecan act as a chelating agent during the reaction process. Furthermore, the structure-designedcathode material shows significantly improved specific capacity and C-rate performance. Theimproved performance is attributed to the novel structure. The graphene and thecoated-carbon can form an electronic conducting network. On the other hand, thecoated-carbon on the surface of Li3V2（PO4）3particles also can partly block the direct contactsbetween electrolyte and LVP nanoparticles, leading to an improved cycling performance inthe wider voltage window. This strategy demonstrated in this paper offers a convenient andeffective technology to design electrode structures for a wide variety of applications inlithium-ion battery or other energy storage systems. |
